Motorized window treatments, such as, for example, motorized roller shades and draperies, provide for control of the amount of sunlight entering a space. Some prior art motorized window treatments have been automatically controlled in response to various inputs, such as daylight sensors and timeclocks, to control the amount of daylight entering a space to adjust the total lighting level in the space to a desired level. For example, the load control system may attempt to maximize the amount of daylight entering the space in order to minimize the intensity of the electrical lighting in the space. In addition, some prior art load control systems additionally controlled the positions of the motorized window treatments to prevent sun glare in the space to increase occupant comfort, for example, as described in greater detail in commonly-assigned U.S. Pat. No. 7,950,827, issued May 31, 2011, entitled ELECTRICALLY CONTROLLABLE WINDOW TREATMENT SYSTEM TO CONTROL SUN GLARE IN A SPACE, the entire disclosure of which is hereby incorporated by reference.
One prior art load control system controlled the position of motorized roller shades to limit the sunlight penetration depth in the space to a maximum penetration depth while minimizing movements of the roller shades to minimize occupant distractions, as described in commonly-assigned U.S. Pat. No. 8,288,981, issued Oct. 16, 2012, entitled METHOD OF AUTOMATICALLY CONTROLLING A MOTORIZED WINDOW TREATMENT WHILE MINIMIZING OCCUPANT DISTRACTIONS, the entire disclosure of which is hereby incorporated by reference. Specifically, the load control system controls the position of the motorized roller shades in response to a calculated position of the sun to thus limit the sunlight penetration depth in the space on sunny days. During a cloudy day, the load control system is operable to stop controlling the motorized window treatments to limit the sunlight penetration depth to the maximum penetration depth and to simply adjust the positions of the motorized window treatments to predetermined positions. For example, the load control system may comprise a photosensor (i.e., a daylight sensor or a radiometer) mounted to a window or to the outside of the building for detecting a cloudy condition. The load control system may detect the cloudy condition, for example, if a total light level measured by the photosensor is below a constant threshold THCONST.
FIGS. 1 and 2 show example plots of the total light level LSENSOR measured by the photosensor on a sunny day and a cloudy day, respectively. On both sunny and cloudy days, the total light level LSENSOR measured by the photosensor increases from zero at sunrise (i.e., at time tSUNRISE) and then begins to decrease toward zero at sunset (i.e., at time tSUNSET). On the cloudy day shown in FIG. 2, the total light level LSENSOR measured by the photosensor does not exceed the constant threshold THCONST, such that the load control system controls the motorized window treatments to predetermined positions (i.e., the load control system will not control the motorized window treatments to limit the sunlight penetration depth to the maximum penetration depth at any point in the day). On the sunny day shown in FIG. 1, the load control system begins to control the motorized window treatments to limit the sunlight penetration depth to the maximum penetration depth when the total light level LSENSOR measured by the photosensor exceeds the constant threshold THCONST at time tENABLE, and then stops controlling the motorized window treatments to limit the sunlight penetration depth to the maximum penetration depth when the total light level LSENSOR measured by the photosensor drops below the constant threshold THCONST at time tDISABLE.
However, on the sunny day near sunrise and sunset as shown in FIG. 1, the load control system may mistakenly conclude that the present day is cloudy when the total light level LSENSOR measured by the photosensor is less than the constant threshold THCONST (i.e., between tSUNRISE and tENABLE and between tDISABLE and tSUNSET). At these times, the sun may be very low in the sky and may shine directly into the windows of the building, thus creating glare conditions. Thus, there is a need for a load control system that is able to more accurately distinguish between sunny and cloudy days in order to prevent glare around sunrise and sunset on sunny days.